Analysis of the Suitability of Mechanics Models for Calculating Interface Surface Tension

Abstract

The suitability of mechanics models for calculating interface surface tension (ST) is analyzed from the viewpoint of molecular kinetic theory. A theory based on the lattice gas model is shown to consider the intermolecular interactions of comparable components and a change in the average bond lengths between dense phase particles in a quasi-chemical approximation that describes direct correlations. It can be applied to three aggregate states and their interfaces, allowing comparison of mechanics and thermodynamics models if the concept of ST is introduced. It is found that the Laplace equation is incompatible with the conditions of the equilibrium of coexisting phases on distorted vapor–liquid interfaces, but it can be used to describe the mechanical equilibrium in systems with an intermediate film between the neighboring phases (if there is no chemical equilibrium between them). Mechanical and thermodynamic definitions of ST under different conditions are discussed. It is shown that to calculate equilibrium ST, we must use the Gibbs definition as the excess free energy at the interface. A procedure for calculating nonequilibrium analogs of surface characteristics (free Helmholtz energy, chemical potential, ST) of solid solutions is formulated that considers internal deformations of solid boundaries and the effect of external loads.